The invention relates to a chain saw.
In a known chain saw of this type (DE 21 32 747 A1), a torsion spring is disposed between the rotating member and the cam, and this torsion spring drives the cam resting against the stop in a rotation direction such that the guide is constantly subjected to a longitudinal displacing force which strives to increase the guide""s distance from the sprocket so that the saw chain is subjected to a constant tension force which compensates for a progressive lengthening of the saw chain due to abrasion and temperature increase during operation. During assembly, the torsion spring is stretched by means of the rotating member embodied as a turning knob until the cam rests against the stop with a sufficient degree of initial stress.
In a likewise known chain saw with a chain tension device (WO 98/33631 A1), the guide that guides the saw chain is fastened between two securing plates and is secured together with them to the housing so that it can be moved in the longitudinal direction of the blade. A bolt that passes through a bore in a cam and an oblong hole in the housing is screwed into the one securing plate, which is guided in a longitudinally movable fashion on the housing, and fixes the cam to the housing by means of its bolt head. In order to stretch the chain tight, the bolt is loosened slightly and the cam, which is supported with its circumference against a housing stop, is rotated so that the bolt moves with the securing plates in the direction of the free end of the blade. The rotation of the cam is stopped when it is evident by feel that the saw chain has reached a sufficiently high tension. The bolt is then tightened again so that the cam and the securing plates are fixed to the housing, with the guide in the adjusted position.
The chain saw according to the invention has the advantage that the detent overload clutch always sets a chain tension that is present at the factory and is independent of the user, and consequently, the user is not presented with the problem of whether the chain tension that he adjusts by feel is also the correct chain tension for an optimal operation of the chain saw. The chain tension device can thus be operated manually without tools which is why it requires only a rotating movement and no other axial movements of the blade whatsoever. An automatic loosening of the chain tension device resulting in decreasing chain tension during operation of the chain saw can be reliably prevented by means of a simple additional measure.
According to an advantageous embodiment of the invention, the cam is supported on the rotating member in a rotating, but axially fixed manner and the detent overload clutch has a first clutch disk provided with detent elements and non-rotatably fixed to the cam and a second clutch disk provided with detent elements and non-rotatably fixed to the rotating member as well as a prestressed clutch spring which presses the two clutch disks axially against each other, with their detent elements engaging one another. With this structural embodiment, the detent coupling is realized in a technically very simple manner and the detent overload force and thus the chain tension can be set by means of the clutch spring that is embodied as a compression spring.
According to a preferred embodiment of the invention, the rotating member is embodied as a clamping sleeve which is screwed with an internally threaded section onto an externally threaded section of a stay rod secured in the housing. In order to actuate the clamping sleeve, a turning knob is slid onto it in a nonrotating, preferably form-fitting manner. This structural embodiment achieves a simple operation of the chain tension device by means of a turning knob.
According to an advantageous embodiment of the invention, the stay rod protrudes through an oblong hole embodied in the guide, while the clamping sleeve overlaps the oblong hole and by being screwed onto the stay rod, pressed against the guide so that it is immovably fixed to the housing after the chain saw is stretched tight. As a result, the set chain tension is reliably maintained during operation of the chain saw.
According to an advantageous embodiment of the invention, the sprocket and the chain tension device are covered by a protective hood which is attached to the housing and permits the damping sleeve to protrude through it, and the turning knob non-rotatably slid onto the clamping sleeve is guided in rotary fashion in the protective hood. Between the turning knob and the protective hood, there is a manually detachable detent mechanism whose form-fitting engagement prevents the turning knob from turning back counter to the rotation direction for screwing the clamping sleeve onto the stay rod. This detent mechanism reliably prevents a loosening of the chain tension device through vibrations during operation of the chain saw.
According to an advantageous embodiment of the invention, the stop on the guide for supporting the cam is embodied as a tensioning piece, which at the one end is slid with a pin in form-fitting manner into a bore in the guide and at the other end, rests with a shaped element in a lateral, form-fitting manner in the oblong hole of the guide and preferably rests against the end of the oblong hole remove from the sprocket.